1. Field of the Invention
This invention relates to a railcar cushioning device, and in particular to a means for creating a cushioning device which is responsive to a preselected force.
2. Description of the Prior Art
Railcars experience a great deal of shock during coupling operations and other train action which can damage cargo on the railcars and the railcars themselves. To absorb the high forces experienced by railcars during these operations, cushioning devices have been employed between the frame of the railcar and its coupler.
The shock experienced by railcars results from both buff and draft forces applied to the coupler of the railcar. The term "buff" is used to describe the movement experienced by a coupler when it is moved towards its associated railcar. These buff forces are usually experienced during coupling operations between the railcars. "Draft" describes the outward movement of the coupler away from its associated railcar in response to pulling forces acting on the coupler.
These cushioning devices are usually hydraulic piston and cylinder arrangements which absorb both buff and draft forces. The cylinders are filled with a hydraulic fluid which is forced through ports in the cylinder wall in response to the impact force applied to the piston. Characteristic of these cushioning devices, however, is the low level of impedance they provide in response to very low buff and draft forces. This response to very low buff and draft forces creates the undesirable effect of "slack" or "slop" between the railcars.
There is a need therefore for a cushioning device for a railcar that operates at a very high impedance when subjected to low level forces to thereby reduce the effect of slack between the railcars while still providing an effective cushion for absorbing high levels of shock between the railcars.